1. Field of the Invention
This invention relates in general to a drying process and its apparatus adapted for use in drying grain, agricultural products, marine products, wood and the like efficiently with a relatively high temperature.
2. Description of the Prior Art
Recently, a refrigeration system has been widely used in an apparatus for drying various kinds of products such as rice, wheat, bean, potato, tabacco, herb, sea food, wood and the like taking notice of the fact that it exhibits the excellent ability of effectively drying air due to dehumidification. Such a drying apparatus is constructed in a manner to install the refrigeration system at the interior or exterior of a drying chamber to dehumidify air in the chamber due to cooling and drying, heat dried air using a suitable heating means such as an electric heater, a condenser or the like, and then return dried and heated air to the drying chamber. The utilization of the refrigeration system as a dryer can exhibit excellent dehumidification of air, and the heating of cooled and dried air can be readily effected by means of a heater or the like. However, such a drying system only keeps the drying chamber at a temperature as high as 30.degree. to 40.degree. C. Accordingly, this system is effective to carry out drying at a relatively low temperature of at most 30.degree. to 40.degree. C., because it can withstand continuous service for a long period of time at such low temperature.
However, as a matter of fact, ideal drying of such products as described above must be carried out by successively keeping the products at a higher temperature of approximately 60.degree. to 80.degree. C. for a long period of time. Therefore, the drying system utilizing refrigeration cycle is not suitable for accomplishing effective drying of the products, because the drying temperature is too low.
Such a disadvantage of the conventional drying system may be overcome by using a suitable heating means such as a heater of a larger capacity to heat the drying chamber to a desired temperature of 60.degree. to 80.degree. C. In this case, the drying chamber is readily heated to the desired temperature. However, when air in the drying chamber heated to the high temperature by the heater is introduced into a refrigeration system as it is without limiting the rate of air to be introduced, an evaporator reduces in function of cooling the air, because the temperature of the air is so high beyond the cooling capacity of the evaporator and the high-temperature air which has failed to be cooled is recycled to the condenser to be re-heated. This allows a temperature of the drying chamber to be increased but causes the dehumidification effect of the drying chamber to be substantially decreased. Thus, a compressor is overloaded because of the evaporator being exposed to a high temperature, and the whole refrigeration system is over the limit of its capacity, resulting in power consumption being too large and the refrigerator system being eventually damaged.
In general, some requirements are to be met in order that the refrigeration system may reliably exhibit its performance and effectively carries out continuous service for a long period of time when it is used for drying. In this regard, it is essential to keep air to be introduced into the evaporator at a temperature below approximately 40.degree. C. If not, such disadvantages as described above are created.
More particularly, dehumidification by means of the evaporator can be effectively carried out as the difference between a temperature of air to be introduced therein and the dew point thereof is large. Supposing that air having a temperature of 30.degree. to 40.degree. C. or less introduced into the evaporator is cooled to about 0.degree. to 5.degree. C. which is near the dew point, moisture in air can be efficiently removed, because the temperature difference therebetween is 30.degree. to 35.degree. C. which is large enough to accomplish dehumidification in the evaporator. On the contrary, supposing that air of about 60.degree. to 80.degree. C. is introduced directly into the evaporator and cooled to about 40.degree. C., dehumidification of the air does not substantially occur even if the temperature difference in air is significantly large, because the temperature of cooled air is far above the dew point.
Further, an apparatus for drying products as described above is continuously used day and night extending over a long period of time. It is possible for a conventional drying apparatus utilizing combustion energy of petroleum to carry out such continuous service, however, the use of petroleum causes the running costs to be extremely high. Whereas, a conventional drying apparatus using a refrigeration cycle involves such overload problem as described above and is not suitable for use in industry at all. Accordingly, there is a strong demand for reliable drying process and apparatus which can allow a refrigeration system to be continuously driven for a long period of time while keeping air in a drying chamber without containing products therein at a temperature in the range of about 60.degree. to 80.degree. C.